1. Field of the Invention
The present invention relates to a steering apparatus for a vehicle.
Priority is claimed on Japanese Patent Application No. 2003-173425, filed Jun. 18, 2003, the content of which is incorporated herein by reference.
2. Description of Related Art
A steering apparatus for a vehicle of a steer-by-wire system (hereinafter referred to as the SBW apparatus) is known in which the steering axis coupled to the steering wheel is mechanically separated from the steering mechanism that steers the steered wheel, and the steering motor provided in the steering mechanism is electrically controlled.
In this SBW apparatus, the target value for the steering angle of a steered wheel is calculated based on the operating amount which the vehicle driver gives to the steering wheel, and the steering motor is feedback-controlled in accordance with the target value so as to control the steering angle of the steered wheel.
This feedback control is basically a position control in which the actual value is made to follow the target value, and the steady-state deviation between the target value and the actual value is generally reduced by providing the feedback system with an integral control device or the like.
Moreover, because in the SBW apparatus the steering wheel and the steered wheel are mechanically separated from each other, it is practically impossible for a vehicle driver to directly feel through the steering wheel the road surface reaction force that is applied to the steered wheel. In the case in which the road surface information is not transmitted from the steered wheel to the steering wheel, if the steered wheel, for example, bumps against a curb and becomes stuck, the driver, without perceiving it, will continue steering, so that the target value for the steering angle of the steered wheel will become greatly excessive, and the steering motor will undergo an excessive load in an effort to follow the target value. In addition, the vehicle driver may have a feeling that something is wrong when he knows, notwithstanding his turning of the steering wheel, that the steered wheel does not change its direction.
As a means to cope with such a disadvantage, a technique is disclosed in Japanese Patent Application Unexamined Publication No. 10-217998, in which a simulated torque of reaction force is generated by an electric motor or the like in accordance with the deviation between the target control amount corresponding to the rotation angle (operating amount) of the steering wheel and the actual steering angle of the steered wheel, i.e., in accordance with the deviation between the target value and the actual value in the feedback control. The simulated torque of reaction force is applied to the steering wheel to give the vehicle driver feedback sensation.
Another technique is disclosed in Japanese Patent Application Unexamined Publication No. 2002-160642, in which in order to improve the responsiveness in control of the steering angle of the steered wheel, the operating torque that acts on the steering wheel is detected, and the control amount of the steering motor is feedforward-controlled in accordance with the operating torque.
However, in the case the integral control is performed to reduce the steady-state deviation between the target value and the actual value, if the target value and the actual value become equal at a certain steering angle, the operating reaction force based on the deviation will cease to be applied, resulting in the loss of a sensation of steering. This is a characteristic not found in the conventional steering apparatus in which the steering wheel and the steered wheel are mechanically coupled, and will give a very strong sensation that something is wrong to a vehicle driver who is accustomed to driving a conventional vehicle.
The present invention has been made in view of the above problems and it is an object thereof to provide a steering apparatus for a vehicle which enables the operating reaction force to be set as desired, and which enables improvement in the responsiveness.